INTERNATIONAL UNION OF PURE AND APPLIED CHEMlSTRY
MACROMOLECULAR DIVISION
COMMISSION ON MACROMOLECULAR NOMENCLATURE*
DEFINITIONS OF BASIC TERMS RELATING TO
LOW-MOLAR-MASS AND POLYMER LIQUID
CRYSTALS
(IUPAC Recommendations 2001)
Prepared for publication by M. BARÓN
Facultad de Ciencias Exactas y Naturales, Universidad de Belgrano, Buenos Aires, Argentina
This document was prepared by a Working Group consisting of:
C. Noël (France); V. P. Shibaev (Russia); M. Barón (Argentina); M. Hess (Germany); A. D. Jenkins (UK); Jung-Il Jin (Korea); A. Sirigu (Italy); R. F. T. Stepto (UK); and W. J. Work (USA); with contri-butions from G. R. Luckhurst (UK); S. Chandrasekhar (India); D. Demus (Germany); J. W. Goodby (UK); G. W. Gray (UK); S. T. Lagerwall (Sweden); O. D. Lavrentovich (USA); M. Schadt (Switzerland), of the International Liquid Crystal Society.
*Membership of the Commission during the preparation of this report (1987–97) was as follows:
Titular Members: G. Allegra (Italy, to 1990); R. E. Bareiss (Germany, to 1994); M. Barón (Argentina, National
Representative from 1988, Associate Member from 1992, Titular Member from 1996); N. M. Bikales (USA, Secretary to 1988); K. Hatada (Japan, Associate Member from 1988, Titular Member from 1990); J. Kahovec (Czech Republic, Associate Member from 1988, Titular Member from 1992); P. Kratochvíl (Czech Republic, Chairman to 1992), E. Maréchal (France, Associate Member from 1992, Titular Member from 1994); W. V. Metanomski (USA, Associate Member from 1988, Titular Member from 1992); I. Mita (Japan, to 1990, Associate Member to 1992); C. Noël (France, to 1994); I. M. Papisov (USSR, to 1988, Associate Member to 1992); V. P. Shibaev (Russia, 1996, Associate Member from 1996); R. F. T. Stepto (UK, Associate Member from 1988, Titular Member from 1990, Chairman from 1992); U. W. Suter (Switzerland, to 1992, Associate Member from 1992); W. J. Work (USA, Associate Member from 1986, Secretary from 1988).
Associate Members contributing to this report: J. V. Alemán (Spain, 1996); A. D. Jenkins (UK, Associate Member
to 1988); M. Hess (Germany, from 1996), J.-I. Jin (Korea, National Representative from 1992, Associate Member from 1994); K. Matyjaszewski (USA, 1992–1996); S. Penczek (Poland, from 1994); S. Tripathy (USA, to 1992); L. Shi (PR China, 1988–1996). Others contributing to this report: H. J. Coles (UK); R. B. Fox (USA); N. A. Platé (Russia, National Representative); A.Sirigu (Italy, National Representative).
Definitions of basic terms relating to
low-molar-mass and polymer liquid crystals
(IUPAC Recommendations 2001)
Abstract: This document is the first published by the IUPAC Commission on Macromolecular Nomenclature dealing specifically with liquid crystals. Because of the breadth of its scope, it has been prepared in collaboration with representa-tives of the International Liquid Crystal Society.
The document gives definitions of terms related to low-molar-mass and poly-mer liquid crystals. It relies on basic definitions of terms that are widely used in the field of liquid crystals and in polymer science. The terms are arranged in five sections dealing with general definitions of liquid-crystalline and mesomorphic states of matter, types of mesophases, optical textures and defects of liquid crys-tals, the physical characteristics of liquid crystals (including electro-optical and magneto-optical properties), and finally liquid-crystal polymers. The terms that have been selected are those most commonly encountered in the conventional structural, thermal, and electro-optical characterization of liquid-crystalline mate-rials.
CONTENTS
1. INTRODUCTION
2. GENERAL DEFINITlONS 3. TYPES OF MESOPHASE 4. TEXTURES AND DEFECTS
5. PHYSICAL CHARACTERISTlCS OF LIQUID CRYSTALS 6. LIQUID-CRYSTAL POLYMERS
7. REFERENCES
8. ALPHABETICAL INDEX OF TERMS
9. GLOSSARY OF RECOMMENDED ABBREVIATIONS AND SYMBOLS
1. INTRODUCTION
This document provides definitions of the basic terms that are widely used in the field of liquid crystals and in polymer science (see refs. 1–39). It is the first publication of the Commission on Macromolecular Nomenclature dealing specifically with liquid crystals.
The recommendations made, resulting from the joint effort of the IUPAC Commission IV.1 Working Party and members of the International Liquid Crystal Society, are concerned with terminol-ogy relating to low-molar-mass and liquid-crystal polymers. Since much of the terminolterminol-ogy is common to both classes of liquid crystals, this document has not been divided into sections dealing separately with these two classes of substances. After some general definitions (Section 2), there are sections deal-ing successively with the structures and optical textures of liquid crystals (Sections 3 and 4), their phys-ical characteristics (including electro-optphys-ical and magneto-optphys-ical properties; Section 5), and finally liq-uid-crystal polymers (Section 6). An alphabetical index of terms and a glossary of recommended sym-bols are provided for the convenience of the reader.
2. GENERAL DEFINITIONS
2.1 mesomorphic state
mesomorphous state
A state of matter in which the degree of molecular order is intermediate between the perfect three-dimensional, long-range positional and orientational order found in solid crystals and the absence of long-range order found in isotropic liquids, gases, and amorphous solids.
Notes:
1. The term mesomorphic state has a more general meaning than “liquid-crystal state” (see Definition 2.2), but the two are often used as synonyms.
2. The term is used to describe orientationally disordered crystals, crystals with molecules in ran-dom conformations (i.e., conformationally disordered crystals), plastic crystals, and liquid crys-tals (see Definition 2.3).
3. A compound that can exist in a mesomorphic state is usually called a mesomorphic compound (see Definition 2.11).
4. A vitrified substance in the mesomorphic state is called a mesomorphic glass and is obtained, for example, by rapid quenching or by crosslinking.
2.2 liquid-crystal state
liquid-crystalline state
Recommended abbreviation: LC state
A mesomorphic state having long-range orientational order and either partial positional order or com-plete positional disorder.
Notes:
1. In the LC state, a substance combines the properties of a liquid (e.g., flow, ability to form droplets) and a crystalline solid (e.g., anisotropy of some physical properties).
2. The LC state occurs between the crystalline solid and the isotropic liquid states on varying, for example, the temperature.
2.2.1 liquid-crystalline phase
Recommended abbreviation: LC phase
A phase occurring over a definite temperature range within the LC state.
2.3 liquid crystal
Recommended abbreviation: LC A substance in the LC state.
Note: A pronounced anisotropy in the shapes and interactions of molecules, or molecular aggre-gates is necessary for the formation of liquid crystals.
2.4 mesophase
A phase occurring over a definite range of temperature, pressure, or concentration within the meso-morphic state.
2.4.1 enantiotropic mesophase
A mesophase that is thermodynamically stable over a definite temperature or pressure range.
2.4.2 thermotropic mesophase
A mesophase formed by heating a solid or cooling an isotropic liquid, or by heating or cooling a ther-modynamically stable mesophase.
Notes:
1. The adjective “thermotropic” describes a change of phase with a change of temperature. “Thermotropic” may also be used to qualify types of mesophase (e.g., thermotropic nematic). 2. Analogous changes can also occur on varying the pressure in which case the mesophase may be
termed barotropic mesophase.
2.4.3 lyotropic mesophase
A mesophase formed by dissolving an amphiphilic mesogen in a suitable solvent, under appropriate conditions of concentration, temperature, and pressure.
Notes:
1. The essential feature of a lyotropic liquid crystal is the formation of molecular aggregates or micelles as a result of specific interactions involving the molecules of the amphiphilic mesogen and those of the solvent.
2. See Definition 2.11.1 for the definition of an amphiphilic mesogen.
3. The mesomorphic character of a lyotropic mesophase arises from the extended, ordered arrange-ment of the solvent-induced micelles. Hence, such mesophases should be regarded as based not on the structural arrangement of individual molecules (as in a nonamphiphilic or a thermotropic mesophase), but on the arrangement within multimolecular domains.
2.4.4 amphitropic compound
A compound that can exhibit thermotropic as well as lyotropic mesophases.
Note: Examples are potassium salts of unbranched alkanoic acids, lecithin, certain polyiso-cyanates, cellulose derivatives with long side-chains, such as (2-hydroxypropyl)cellulose, and cyanobiphenyl derivatives of alkyl(triethyl)ammonium bromide.
2.4.5 monotropic mesophase
A metastable mesophase that can be formed by supercooling an isotropic liquid or an enantiotropic mesophase at a given pressure to a temperature below the melting point of the crystal.
Note: Monotropic transition temperatures (see Definition 2.5.) are indicated by placing parenthe-ses, (), around the values.
2.5 transition temperature
Recommended symbol: TXY SI unit: K
The temperature at which the transition from mesophase X to mesophase Y occurs.
Note: Mesophase X should be stable at lower temperatures than phase Y. For example, the nemat-ic-isotropic transition temperature would be denoted as TNI.
2.6 clearing point
clearing temperature isotropization temperature
Recommended symbol: Tclor Ti SI Unit: K
The temperature at which the transition between the mesophase with the highest temperature range and the isotropic phase occurs.
2.7 virtual transition temperature
A transition temperature that cannot be measured directly, determined by extrapolation of transition lines in binary phase diagrams to 100% of that particular component.
Notes:
1. A virtual transition temperature lies outside the temperature range over which the (meso) phase implied can be observed experimentally.
2. A virtual transition temperature is not well defined; it will, for example, depend on the nature of the liquid-crystal components used to construct the phase diagram.
3. A virtual transition temperature is indicated by placing square brackets, [ ], around its value.
2.8 transitional entropy
Recommended symbol:∆SXY SI unit: J K–1 mol–1
The change in entropy on transition from phase X to phase Y. Notes:
1. The transitional entropy reflects the change in order, both orientational and translational, at the phase transition.
2. Phase X should be stable at lower temperatures than phase Y.
3. Numerical values of the molar transitional entropy should be given as the dimensionless quanti-ty ∆SXY/R where R is the gas constant.
2.9 divergence temperature
pretransitional temperature
Recommended symbol: T* SI Unit: K
The temperature at which the orientational correlations in an isotropic phase diverge. Notes:
1. The divergence temperature is the lowest limit of metastable supercooling of the isotropic phase. 2. The divergence occurs at the point where the isotropic phase would be expected to undergo a sec-ond-order transition to the liquid-crystal phase, were it not for the intervention of a first-order transition to the liquid-crystal phase.
3. The divergence temperature for nematogens can be measured by using the Kerr effect or Cotton–Mouton effect or by light-scattering experiments.
4. T* occurs below the clearing temperature, usually by about 1 K in isotropic-to-nematic transitions and increases to at least 10 K for isotropic-to-smectic transitions.
2.10 mesogenic group
mesogenic unit mesogenic moiety
A part of a molecule or macromolecule endowed with sufficient anisotropy in both attractive and repul-sive forces to contribute strongly to LC mesophase, or, in particular, to LC mesophase formation in low-molar-mass and polymeric substances.
Notes:
1. “Mesogenic” is an adjective that in the present document applies to molecular moieties that are structurally compatible with the formation of LC phases in the molecular system in which they exist.
2. Mesogenic groups occur in both low-molar-mass and polymeric compounds.
Examples of mesogenic groups are
where X and Y are covalent bonds or linking units such as:
2.11 mesogen
mesogenic compound mesomorphic compound
A compound that under suitable conditions of temperature, pressure, and concentration can exist as a mesophase, or, in particular as a LC phase.
Notes:
1. When the type of mesophase formed is known, more precisely qualifying terminology can be used, e.g., nematogen, smectogen, and chiral nematogen.
2. When more than one type of mesophase can be formed, more than one qualification could apply to the same compound, and then the general term mesogen should be used.
2.11.1 amphiphilic mesogen
A mesogen composed of molecules consisting of two parts of contrasting character that are hydrophilic and hydrophobic or lipophobic and lipophilic.
Notes:
1. Examples of amphiphilic mesogens are soaps, detergents, and some block copolymers.
2. Under suitable conditions of temperature and concentration, the similar parts of amphiphilic mol-ecules cluster together to form aggregates or micelles (see Definition 2.4.2).
2.11.2 nonamphiphilic mesogen A mesogen that is not of the amphiphilic type.
Notes:
1. At one time it was thought that a nonamphiphilic molecule had to be long and rod-like for mesophase formation, but it has now been established that molecules of other types and shapes, for example, disc-like and banana-shaped molecules, may also form mesophases. (See ref. 6). 2. A selection of the types of nonamphiphilic mesogens is given in definitions 2.11.2.1.–2.11.2.8.
2.11.2.1 calamitic mesogen
A mesogen composed of rod- or lath-like molecules. Note: Examples are:
• 4-butyl-N-(4-methoxybenzylidene)aniline (BMBA) (a) • 4,4´-dimethoxyazoxybenzene (b)
• 4-(trans-4-pentylcyclohexyl)benzonitrile (d)
• cholesterol and cholest-5-ene-3-carboxylic acid esters (e).
2.11.2.2 discotic mesogen discoid mesogen
A mesogen composed of relatively flat, disc- or sheet-shaped molecules. Notes:
1. Examples are: hexa(acyloxy)benzenes (a), hexa(acyloxy)- and hexa-alkyloxytriphenylenes (b), 5H,10H,15H-diindeno[1,2-a:1´,2´-c]fluorene derivatives (c).
Examples of some appropriate substituent groups are:
2. The adjective “discotic” is also employed to describe the nematic mesophases formed by discot-ic mesogens. The mesophases formed by a columnar stacking of disc-like molecules are described as columnar mesophases (see Definitions 3.2).
2.11.2.3 pyramidic mesogen
conical or cone-shaped mesogen bowlic mesogen
A mesogen composed of molecules containing a semi-rigid conical core.
2.11.2.4 sanidic mesogen
A mesogen composed of board-like molecules with the long-range orientational order of the phase reflecting the symmetry of the constituent molecules.
Note: See also Definition 3.4.
2.11.2.5 polycatenary mesogen
A mesogen composed of molecules each having an elongated rigid core with several flexible chains attached to the end(s).
Notes:
1. The flexible chains are usually aliphatic.
2. The numbers of flexible chains at the ends of the core can be indicated by using the term
m,n-polycatenary mesogen.
3. There exist several descriptive names for these mesogens. Examples are: (a) biforked mesogen (2,2-polycatenary mesogen); (b) hemiphasmidic mesogen (3,1-polycatenary mesogen); (c) forked hemiphasmidic mesogen (3,2-polycatenary mesogen); and (d) phasmidic mesogen (3,3-polycatenary mesogen). Examples of each type with the core represented by
are given together with a specific example of a forked hemiphasmidic mesogen (c).
A specific example of (c) is
2.11.2.6 swallow-tailed mesogen
A mesogen composed of molecules each with an elongated rigid core with, at one end, a branched flex-ible chain, having branches of about the same length.
and an example is the fluorene derivative
2.11.2.7 bis-swallow-tailed mesogen
A mesogen composed of molecules each with an elongated rigid core and a branched flexible chain, with branches of about the same length, attached at each end.
Example:
2.11.2.8 laterally branched mesogen
A mesogen composed of rod-like molecules with large lateral branches such as alkyl, alkoxy, or ring-containing moieties.
Example:
2.11.2.9 liquid-crystal oligomer mesogenic oligomer
A mesogen constituted of molecules, each with more than one mesogenic group. Notes:
1. The mesogenic groups usually have identical structures.
2. A liquid-crystal dimer or mesogenic dimer is sometimes known as a twin mesogen. Use of the terms “dimesogenic compounds” and “Siamese-twin mesogen” for “liquid-crystal dimer” or “mesogenic dimer” is not recommended.
A specific example of type (c), a tail-to-tail liquid-crystal dimer, is
wherein –[CH2]8– is the flexible spacer linking the two mesogenic groups.
4. A liquid-crystal dimer with different mesogenic groups linked by a spacer is known as an asym-metric liquid-crystal dimer.
5. A liquid-crystal dimer with flexible hydrocarbon chains having an odd number of carbon atoms is called an odd-membered liquid-crystal dimer, while one with hydrocarbon chains having an even number of carbon atoms is called an even-membered liquid-crystal dimer.
2.11.2.10 banana mesogen
A mesogen constituted of bent or so-called banana-shaped molecules in which two mesogenic groups are linked through a semi-rigid group in such a way as not to be colinear.
Note: Examples of such structures are
with the substituent group R being an alkyl ether (–OCnH2n+1) 2.11.3 metallomesogen
A mesogen composed of molecules incorporating one or more metal atoms. Notes:
1. Metallomesogens may be either calamitic (see Definition 2.11.2.1) or discotic (see Definition 2.11.2.2).
3. TYPES OF MESOPHASE
3.1 mesophases of calamitic mesogens
3.1.1 uniaxiaI nematic mesophase nematic
Recommended symbol: N or Nu
A mesophase formed by a nonchiral compound or by the racemate of a chiral compound in which the spatial distribution of the molecular centers of mass is devoid of long-range positional order and the molecules are, on average, orientationally ordered about a common axis defined as the director and rep-resented by the unit vector n.
Notes:
1. See Fig. 1 for an illustration of the molecular organization in a uniaxial nematic mesophase. 2. The unit vector, n, is defined in 3.1.1.1 (see also Fig. 1).
3. The direction of n is usually arbitrary in space.
4. The extent of the positional correlations for the molecules in a nematic phase is comparable to that of an isotropic phase, although the distribution function is necessarily anisotropic.
5. From a crystallographic point of view, the uniaxial nematic structure is characterized by the sym-bol D∞hin the Schoenflies notation (∞/mm in the International System).
6. Since the majority of nematic phases are uniaxial, if no indication is given, a nematic phase is assumed to be uniaxial but, when there is the possibility of a biaxial as well a uniaxial nematic, a uniaxial phase should be denoted as Nu(see Definition 3.3.1).
3.1.1.1 director
Recommended symbol: n
The local symmetry axis for the singlet, orientational distribution of the molecules of a mesophase. Notes:
1. The director is defined as a unit vector, but directions +n and –n are arbitrary.
2. In uniaxial nematics, formed by compounds consisting of either rod- or disc-like molecules, the mean direction of the effective molecular symmetry axis coincides with the director.
3. The director also coincides with a local symmetry axis of any directional property of the mesophase, such as the refractive index or magnetic susceptibility.
3.1.2 cybotactic groups
An assembly of molecules in a nematic mesophase having a short-range smectic-like array of the con-stituent molecules.
Note: Two types of short-range smectic-like structures are possible. One is analogous to a smec-tic A mesophase where the molecules tend to lie along a layer normal (see Definition 3.1.5.1.1), and the other is like a smectic C mesophase (see Definition 3.1.5.1.2) where the molecules tend to be oblique with respect to a layer normal. See Fig. 2 for illustrations of the molecular arrangements in the smectic A-type structure and the smectic C-type structure.
3.1.3 chiral nematic mesophase chiral nematic
cholesteric mesophase cholesteric
Recommended symbol: N*
A mesophase with a helicoidal superstructure of the director, formed by chiral, calamitic, or discotic molecules or by doping a uniaxial nematic host with chiral guest molecules in which the local director
n precesses around a single axis.
Fig. 2 Schematic representation of the molecules in (a) a smectic A-like local structure and (b) a smectic C-like
local structure, making angle ϑwith the layer normal.
Notes:
1. See Fig. 3 for an illustration of the helicoidal molecular distribution in a chiral nematic mesophase.
2. Locally, a chiral nematic mesophase is similar to a uniaxial nematic, except for the precession of the director n about the axis, Z.
3. The director is periodic along Z with the pitch P of the helical structure equal to a turn of the local director n by 2π.
4. Chiral nematic mesophases exhibit Bragg scattering of circularly polarized light at a wavelength λRproportional to the pitch P (λR= <n>P, where <n> is the mean refractive index).
5. The director precession in a chiral nematic mesophase is spontaneous and should be distinguished from an induced twisted structure produced by a mechanical twist of a nematic mesophase between confining surfaces.
6. The term chiral nematic mesophase or chiral nematic is preferred to cholesteric or cholesteric mesophase.
3.1.4 blue phase
Recommended abbreviation: BP
A mesophase with a three-dimensional spatial distribution of helical director axes leading to frustrated structures with defects arranged on a lattice with cubic symmetry and lattice constants of the order of the wavelength of visible light.
Notes:
1. See Fig. 4 for a possible model for a BP.
2. The name “blue phase” derives historically from the optical Bragg reflection of blue light but, because of larger lattice constants, BPs can reflect visible light of longer wavelengths.
3. With chiral nematic substances forming chiral nematic mesophases of short pitch (<700 nm), up to three blue phases occur in a narrow temperature range between the chiral nematic phase and the isotropic phase.
4. A BP is optically isotropic and exhibits a Bragg reflection of circularly polarized light.
5. Two BPs of different cubic symmetry (space group I 4132 for BP I and P 4232 for BP II) are presently known, together with a third (BPIII) of amorphous structure. Several other BPs of dif-ferent cubic symmetry exist but only in the presence of external electric fields.
3.1.5 smectic mesophase
Recommended abbreviation: Sm
A mesophase that has the molecules arranged in layers with a well-defined layer spacing or periodici-ty.
Notes:
1. There are several types of smectic mesophases, characterized by a variety of molecular arrange-ments within the layers.
2. Although the total number of smectic mesophases cannot be specified, the following types have been defined: SmA, SmB, SmC, SmF, and SmI. The alphabetical order of suffixes merely indi-cates an order of discovery.
3. The classification of SmD as smectic is largely a consequence of history, and should be discon-tinued (see Definition 3.1.9).
4. At one time, a number of mesophases were identified as smectic on the basis of their optical tex-tures, but they are in fact soft crystals characterized by very low yield stresses. Hence, these three-dimensionally ordered phases should no longer be called smectic mesophases. They are akin to plastic crystals with some elementary long-range order and are referred to by the letters E, J, G, H, K (see 3.1.5.3).
5. Tilted smectic mesophases formed by chiral compounds or containing chiral mixtures are desig-nated by the superindex * (SmC*, SmF*, etc.). (See, e.g., Definition 3.1.5.1.3.)
3.1.5.1 smectic mesophases with unstructured layers 3.1.5.1.1 smectic A mesophase
Recommended abbreviation: SmA
A smectic mesophase involving a parallel arrangement of the molecules within layers, in which the long axes of the molecules tend to be perpendicular; the layer planes and the molecular centers of mass have no long-range positional order parallel to the layer planes
Notes:
1. See Fig. 5 for the molecular organization in a smectic A mesophase.
2. Each layer approximates to a true two-dimensional liquid. The system is optically uniaxial, and the optic axis, Z, is normal to the layer planes.
3. The directions +Z and –Z are interchangeable.
4. The structure of a smectic A mesophase is characterized by the symbol D∞hin the Schoenflies notation (∞, 2 in the International System).
5. The lyotropic equivalent of a smectic A mesophase is known as a lamellar mesophase; where layers of amphiphilic molecules are separated by layers of solvent, normally water, or by oil in an inverse lamellar mesophase.
6. A smectic A-phase containing a chiral molecule or dopant, can be called a chiral smectic A-phase. The recommended symbol is SmA* wherein the (*) indicates that the macroscopic structure of the mesophase is chiral.
3.1.5.1.2 smectic C mesophase
Recommended abbreviation: SmC
The analog of a smectic A mesophase involving an approximately parallel arrangement of the mole-cules within layers in which the director is tilted with respect to the layer normal and the molecular cen-ters-of-mass have no long-range positional order parallel to the layer planes (see Fig. 6).
Notes:
1. See Fig. 6 for an illustration of the molecular organization in a smectic C mesophase. 2. The physical properties of a smectic C mesophase are those of a biaxial crystal.
3. The smectic C structure corresponds to monoclinic symmetry characterized by the symbol C2h, in the Schoenflies notation and the space group t 2/m in the International System.
4. The tilt direction varies in a random manner from layer to layer in conventional smectic C mesophases. However, it can alternate from layer to layer, as in an antiferro-electric chiral smectic C mesophase (see Definition 5.9, Note 7) and in the smectic C mesophase formed by certain liquid crystal dimers with an odd number of carbon atoms in the spacers (see Definition 2.11.2.9). The recommended symbol for this type of mesophase is SmCa.
3.1.5.1.3 chiral smectic C mesophase Recommended abbreviation: SmC*
A smectic C mesophase in which the tilt direction of the director in each successive layer is rotated through a certain angle relative to the preceding one so that a helical structure of a constant pitch is formed.
Notes:
1. See Fig. 7 for an illustration of the molecular organization in a chiral smectic C mesophase. 2. The (*) in SmC* and analogous notations indicates, as in 3.1.5.1.2 (Note 6), that the
macroscop-ic structure of the mesophase is chiral. However, it is also used simply to indmacroscop-icate that some of the constituent molecules are chiral even though the microscopic structure may not be.
3. A SmC* mesophase is formed by chiral compounds or mixtures containing chiral compounds. 4. Locally, the structure of the chiraI smectic C mesophase is essentially the same as that of the
achi-ral smectic C mesophase except that there is a precession of the tilt direction about a single axis. It has the symmetry C2in the Schoenflies notation.
5. This chiral smectic C phase is also known as the ferro-electric chiral smectic C phase.
6. The helix can be unwound by surface forces to give a surface-stabilized SmC*, which has a macroscopic polarization.
3.1.5.2 hexatic smectic mesophase
A smectic mesophase with in-plane short-range positional molecular order, weakly coupled two-dimen-sional layers and long-range bond orientational molecular order.
Note: There are three types of hexatic smectic mesophases: smectic B (SmB), smectic F (SmF), and smectic I (SmI). Here, the term “hexatic” may be omitted because it is implicit for this group of smectic mesophases.
3.1.5.2.1 smectic B mesophase
Recommended abbreviation: SmB
A hexatic smectic mesophase in which the director is perpendicular to the layers with the long-range hexagonal bond-orientational order.
Notes:
1. See Fig. 8 for an illustration of the molecular organization in a smectic B mesophase.
2. Positional molecular order does not propagate over distances larger than a few tens of nanome-ters but bond orientational molecular order extends over macroscopic distances within and across layers.
3. By contrast with a smectic B mesophase, a crystal B mesophase has correlations of positional order (hexagonal) in three dimensions, i.e., correlations of position occur within and between lay-ers.
4. The structure of a smectic B mesophase is characterized by a D6hpoint group symmetry, in the Schoenflies notation, by virtue of the bond orientational order.
5. A smectic B mesophase is optically uniaxial.
6. A smectic B mesophase is sometimes denoted SmBhex. The subscript “hex” denotes the hexago-nal structure of the mesophase.
3.1.5.2.2 smectic F mesophase
Recommended abbreviation: SmF
A hexatic smectic mesophase the structure of which may be regarded as a C-centered monoclinic cell with a hexagonal packing of the molecules with the director tilted, with respect to the layer normals, toward the sides of the hexagons.
Notes:
1. See Fig. 9a for an illustration of the molecular organization in a smectic F mesophase, a tilted ana-log of the smectic B mesophase.
2. A SmF mesophase is characterized by in-plane short-range positional correlations and weak or no interlayer positional correlations.
3. Positional molecular order extends over a few tens of nanometers but the bond orientational molecular order is long-range within a layer.
4. The point-group symmetry is C2h(2/m) in the Schoenflies notation, and the space group, t 2/m in the International System.
4. The smectic F mesophase is optically biaxial.
5. Chiral materials form chiral smectic F mesophases denoted by SmF*.
3.1.5.2.3 smectic I mesophase
Recommended abbreviation: SmI
A hexatic smectic mesophase the structure of which may be regarded as a C-centered monoclinic cell with hexagonal packing of the molecules with the director tilted, with respect to the layer normals, toward the apices of the hexagons.
Notes:
1. See Figs. 9a and 9b for illustrations of the molecular organizations of smectic F and I mesophas-es. They are tilted analogs of the smectic B mesophase.
2. The smectic I mesophase is optically biaxial.
3. The in-plane positional correlations in a smectic I mesophase are slightly greater than in a smec-tic F mesophase.
4. Chiral materials form chiral smectic I mesophases denoted by SmI*.
3.1.5.3 crystal B, E, G, H, J, and K mesophases
Soft crystals that exhibit long-range positional molecular order, with three-dimensional stacks of layers correlated with each other.
Notes:
1. Originally, these mesophases were designated as smectic, but further investigations have demon-strated their three-dimensional character.
2. In the crystal B and E mesophases, the molecular long axes are essentially parallel to the normals to the layer planes, while in the G, H, J, and K mesophases they are tilted with respect to the layer normals.
3. The E, J, and K phases have herringbone organizations of the molecular short axes, and so the mesophases are optically biaxial.
3.1.6 polymorphic modifications of strongly polar compounds 3.1.6.1 re-entrant mesophase
Recommended subscript: re
The lowest temperature mesophase of certain compounds that exhibit two or more mesophases of the same type, over different temperature ranges.
Notes:
1. Re-entrant mesophases are most commonly observed when the molecules have strong longitudi-nal dipole moments (see example).
2. Sequences of re-entrant mesophases have also been found in binary mixtures of nonpolar liquid-crystalline compounds.
Example: The following compound exhibits, as temperature decreases, an isotropic (I) phase, nematic (N), smectic A (SmA) re-entrant nematic (Nre), re-entrant smectic A (SmAre) mesophases, and a crystalline (Cr) phase, with transitions at the specified temperatures.
Fig. 9 Illustrating the tilt directions of the director in (a) SmF and (b) SmI mesophases indicating, respectively,
3.1.6.2 smectic A1, A2, Ad, C1, C2, and Cd
Recommended abbreviations: SmA1, SmA2, SmAd, SmC1, SmC2, SmCd
Smectic A and smectic C mesophases characterized by antiparallel (SmA2, Sm Ad, and SmC2, Sm Cd) and random (SmA1and SmC1) alignments of the molecular dipoles within the layer thickness in Fig. 10.
Notes:
1. See Figs. 10 and 11 for illustrations of the molecular arrangements in the mesophases.
2. The subscripts 1, d, and 2 indicate that the layer thickness is one, d, and two times the fully extended molecular length, with 1 < d < 2.
3. SmAd and SmCdmesophases form bilayers with partial overlapping of the molecules of adjacent layers.
4. SmA2and SmC2phases form bilayers with antiparallel ordering of the molecules. 5. Bilayer structures are also known for SmB and crystal E mesophases.
Fig. 10 Illustrating the molecular structures of SmAd, SmA1, and SmA2mesophases.
3.1.6.3 modulated smectic mesophase Recommended mark: ~
A smectic mesophase that has a periodic in-plane density variation. Notes:
1. See Figs. 12a and 12b for illustrations of the molecular arrangements in
2. The Smà mesophase is also known as a centered rectangular mesophase or antimesophase. The dimensional space group is cmm in the International System.
3. The SmC~mesophase is also known as an oblique or ribbon mesophase. The dimensional space group is pmg in the International System.
3.1.7 intercalated smectic mesophase Recommended subscript: c
A smectic mesophase is a mesophase that has a spacing between layers (smectic periodicity) of approx-imately one-half of the molecular length.
Notes:
1. Intercalated smectic mesophases are commonly observed for liquid-crystal dimers.
2. At present intercalated smectic A (SmAc) and smectic C (SmCc) as well as intercalated crystal B (Bc), G (Gc), and J (Jc) mesophases have been observed.
3. The local structure in the nematic mesophase of certain dimers exhibit an intercalated smectic mesophase.
3.1.8 induced mesophase
A particular mesophase formed by a binary mixture, the components of which do not separately form mesophases, with the particular mesophase existing above the melting points of both components.
Notes:
1. The formation of an induced mesophase usually results from an attractive interaction between unlike species, the strength of which exceeds the mean of the strengths of the interactions between like species.
2. Examples of such interactions that have been noted are dipolar/nonpolar, charge-transfer, and quadrupolar.
3. Mesophases can also be induced when the free-volume between the large, irregular molecules of one component is filled by the smaller molecules of the second component. Such mesophases have been called filled smectic mesophases, although the term “induced” is recommended. 4. A monotropic mesophase can be stabilized in a mixture when, as a result of melting-point
depres-sion, a metastable mesophase becomes stabilized. Such a mesophase is distinct from an induced mesophase.
3.1.9 cubic mesophase
Recommended abbreviation: Cub
A mesophase with an overall three-dimensional order of cubic symmetry in which each micellar unit cell contains several hundred molecules in random configurations, as in a liquid.
Notes:
1. The mesophase formerly designated as smectic D (see Definition 3.1.5, Note 3) belongs to this class.
2. A cubic mesophase is optically isotropic; it may be distinguished from an isotropic liquid or a homeotropic phase by the fact that the optically black isotropic phase or homeotropic phase nucleates in the birefringent SmC phase in straight-edged squares, rhombi, hexagons, and rectan-gles.
3. A cubic mesophase may be formed by rod-like molecules with strong, specific intermolecular interactions, such as hydrogen bonding, between them. However, they are also found in poly-catenary compounds (see Definition 2.11.2.5) where there are no specific, strong interactions. 4. Cubic mesophases have long been known in thermotropic salt-like compounds and in lyotropic
liquid-crystals.
5. There are several types of thermotropic and lyotropic cubic mesophases, with different symme-try and miscibility properties; when the space groups of these are known, they should be includ-ed in parentheses after the term “Cub”.
Example: The following compound exhibits a crystalline phase (Cr), smectic SmC, cubic (Cub), smectic SmA mesophases, and an isotropic (I) phase, with transitions at the specified temperatures:
3.2 mesophases of disc-like mesogens
discotic mesophases discotics
3.2.1 discotic nematic mesophase discotic nematic
Recommended symbol: N
A nematic mesophase in which disc-shaped molecules, or the disc-shaped portions of macromolecules, tend to align with their symmetry axes parallel to each other and have a random spatial distribution of their centers of mass.
Notes:
1. See Fig. 13 for an illustration of the molecular arrangement in a discotic mesophase.
2. The symmetry and structure of a nematic mesophase formed from disc-like molecules is identi-cal to that formed from rod-like molecules. It is recommended therefore, that the subscript “D” is removed from the symbol “ND”, often used to denote a nematic formed from disc-like molecules. 3. In some cases, the discotic nematic mesophase is formed by compounds that do not have mole-cules of discotic shape (for example, phasmidic compounds, salt-like materials, and oligosaccha-rides).
4. Chiral discotic nematic mesophases, N*, also exist. 3.2.2 columnar mesophase
columnar discotic mesophase columnar discotic
Recommended abbreviation: Col
A mesophase in which disc-shaped molecules, the disc-shaped moieties of macromolecules, or wedge-shaped molecules assemble themselves in columns packed parallel on a two-dimensional lattice, but without long-range positional correlations along the columns.
Note: Depending on the order in the molecular stacking in the columns and the two-dimensional lattice symmetry of the columnar packing, the columnar mesophases may be classified into three major classes: hexagonal, rectangular, and oblique (see Definitions 3.2.2.1 to 3.2.2.3).
3.2.2.1 columnar hexagonal mesophase Recommended abbreviation: Colh
A columnar mesophase characterized by a hexagonal packing of the molecular columns. Notes:
1. See Fig. 14 for an illustration of the molecular arrangement in a Colhmesophase.
2. Hexagonal mesophases are often denoted Colhoor Colhdwhere h stands for hexagonal and o and d refer to the range of positional correlations along the column axes: o stands for ordered and d for disordered. The use of the subscripts o and d should be discontinued. In both cases, the order-ing is liquid-like; only the correlation lengths are different.
3. The relevant space group of a Colh mesophase is P 6/mmm (equivalent to P 6/m 2/m in the International System and point group D6hin the Schoenflies notation).
4. The lyotropic equivalent of a columnar hexagonal mesophase is known as a hexagonal mesophase; in it, columns of amphiphilic molecules are surrounded by the solvent, normally water, or an oil in an inverse hexagonal mesophase.
3.2.2.2 columnar rectangular mesophase Recommended symbol: Colr
A columnar mesophase characterized by a liquid-like molecular order along the columns, in which the columns are arranged in a rectangular packing.
Notes:
1. See Figs. 15a–c for illustrations of molecular arrangements in columnar rectangular mesophases. 2. The average orientation of the planes of the molecular discs is not necessarily normal to the
col-umn axes.
3. Depending on the plane space-group symmetries, three rectangular mesophases are distinguished (see Figs. 15a–c).
4. There also exist chiral columnar rectangular mesophases, with the molecular discs tilted periodi-cally in the columns and with the tilt directions changing regularly down the columns.
3.2.2.3 columnar oblique mesophase Recommended symbol: Colob
A columnar mesophase characterized by a liquid-like molecular order along the column, in which the columns are arranged with an oblique packing.
Notes:
1. See Fig. 15d for an illustration of the molecular arrangement in a columnar oblique mesophase. 2. The average of the planes of the molecular discs is not necessarily normal to the columnar axes. 3. The plane space-group symmetry of a Colobmesophase is P1(see Fig. 15d).
4. There also exist chiral columnar oblique mesophases, with the tilt directions of the columnar discs varying regularly along the columns.
3.3 biaxial mesophase
Recommended subscript: b
A mesophase composed of board-like molecules in which there are long-range orderings of both the longer and the shorter molecular axes.
Fig. 15 Plan views of the two-dimensional lattice of the columns in columnar rectangular (a) to (c) and oblique
Notes:
1. A biaxial mesophase has three orthogonal directors denoted by the unit vectors l, m, and n. 2. The tensorial properties of a biaxial mesophase have biaxial symmetry unlike the uniaxial
sym-metries of, for example, the nematic and smectic A mesophases.
3. The biaxiality of the phase does not result from tilted structures as, for example, in a smectic C mesophase.
4. Distinct biaxial mesophases are created when the molecular centers of mass are correlated with-in the layers. Such mesophases have been proposed for board-like polymers and have been called sanidic mesophases (see Definitions 3.4, 3.4.1, and 3.4.2).
5. Sanidic structures are analogous to the columnar mesophases formed by disc-like molecules (see Definition 3.2.2).
3.3.1 biaxial nematic mesophase biaxial nematic
Recommended symbol: Nb
A mesophase in which the long axes of the molecules are, on average, orientationally ordered about a common director and one of the shorter molecular axes is ordered, on average, about a second, orthog-onal director.
Notes:
1. See Fig. 16 for an illustration of the molecular arrangement in a Nb mesophase.
2. From a crystallographic point of view, the biaxial nematic structure is characterized by the sym-bol D2hin the Schoenflies notation (2/m, m in the International System).
3. In lyotropic systems, biaxial nematic mesophases have been identified from the biaxial symme-try of their tensorial properties.
4. The situation for thermotropic calamitic systems is less clear and for some compounds claimed to form a Nb, detailed investigations have found the mesophase to be of type Nu(see Definition 3.1.1).
5. A biaxial nematic has the same structure as a disordered sanidic mesophase (see Definition 3.4, Note 2); it is recommended that the latter name be discontinued and the name biaxial nematic be used.
3.3.2 biaxial smectic A mesophase Recommended symbol: SmAb
A smectic A mesophase composed of board-like molecules with the longer and the shorter molecular axes orientationally ordered.
Note: For a SmAb mesophase, the molecular centers-of-mass have only short-range positional order within a layer.
3.4 sanidic mesophase
Recommended symbol:Σ
A mesophase in which board-shaped molecules assemble in stacks packed parallel to one another on a one- or two-dimensional lattice (see Figs. 17 and 18).
Notes:
1. See Figs. 17 and 18 for examples of sanidic mesophases.
2. Short board-like shaped molecules usually form biaxial nematic mesophases. It is recommended that the use of the term “disordered sanidic mesophases” for such mesophases be discontinued (see Definition 3.3.1, Note 5).
3. Rotation of the molecules around their long axes is considerably hindered.
3.4.1 rectangular sanidic mesophase Recommended symbol:Σr
A sanidic mesophase in which the molecular stacks are packed regularly side-by-side with long-range order along a stack normal as well as along the long stack-axis.
Note: See Fig. 17 for an illustration of the molecular arrangement in a Σrmesophase.
3.4.2 ordered sanidic mesophase Recommended symbol:Σo
A mesophase in which the molecular stacks are packed regularly side-by-side with long-range order along a stack normal and no registration along the long stack-axis.
Note: See Fig. 18 for an illustration of the molecular arrangement in a Σomesophase.
Fig. 17 Illustrating a rectangular sanidic mesophase.
3.5 glassy mesophase
Recommended subscript: g
A mesophase in which nonvibratory molecular motion is frozen by supercooling a mesophase stable at a higher temperature.
3.6 twist grain-boundary mesophase
Recommended abbreviation: TGB
A defect-stabilized mesophase created when a smectic A mesophase is subjected to a twist or bend dis-tortion.
Notes:
1. The twist and bend distortions can be stabilized by an array of screw or edge dislocations. 2. A TGB mesophase is analogous to the Abrikosov flux-phase of certain superconductors.
3.6.1 twist grain-boundary A* mesophase Recommended abbreviation: TGBA*
A mesophase with a helicoidal supramolecular structure in which blocks of molecules, with a local structure of the smectic A type, have their layer normals rotated with respect to each other and are sep-arated by screw dislocations.
Notes:
1. See Fig. 19 for an illustration of the molecular arrangement of a TGBA* mesophase. 2. The TGBA* mesophase is formed by a chiral compound or a mixture of chiral compounds. 3. Two TGBA* structures are possible; in one, the number of blocks corresponding to a rotation of
the layer normal by 2p is an integer, while in the other, it is a noninteger.
4. A TGBA* is found in a phase diagram between smectic A and chiral nematic mesophases or between a smectic A mesophase and an isotropic phase.
5. The temperature range of existence of a TGBA* mesophase is typically several K.
3.6.2 twist grain-boundary C* mesophase Recommended abbreviation: TGBC*
A mesophase of helicoidal supermolecular structure in which blocks of molecules with a local structure of the smectic C type, have their layer normals rotated with respect to each other and are separated by screw dislocations.
Notes:
1. Two forms of TGBC* mesophase are possible: in one form the director within the layer is tilted and rotates coherently through the layers in a block as in a chiral smectic C mesophase, while in
the other form the director within a block is simply tilted with respect to the layer normal as in a smectic C mesophase.
2. In the case of a short pitch, when P is less than the wavelength λ, the macroscopic extraordinary axis for the refractive index is orthogonal to the director.
3.6.3 melted-grain-boundary mesophase Recommended abbreviation: MGBC*
A mesophase with a helicoidal supramolecular structure of blocks of molecules with a local smectic C structure. The layer normal to the blocks rotates on a cone to create a helix-like director in the smec-tic C*. The blocks are separated by plane boundaries perpendicular to the helical axis. At the boundary, the smectic order disappears but the nematic order is maintained. In the blocks the director rotates from one boundary to the other to allow the rotation of the blocks without any discontinuity in the thermo-molecular orientation.
Note: This phase appears between the TGBA and SmC* or N*and SmC* mesophases.
4. TEXTURES AND DEFECTS
4.1 domain
A region of a mesophase having a single director. Note: See 3.1.1.1 for the definition of a director.
4.2 monodomain
A region of a uniaxial mesophase or a whole uniaxial mesophase having a single director or a region of a biaxial mesophase or a whole biaxial mesophase having two directors.
Notes:
1. See 3.1.1 for the definition of a uniaxial nematic mesophase, 5.8.1 for the definition of uniaxial mesophase anisotropy, and Definitions 3.3 and 5.8.2 relating to biaxial mesophases.
2. For a smectic mesophase, the term “monodomain” also implies a uniform arrangement of the smectic layers.
4.3 homeotropic alignment
A molecular alignment of which the director is perpendicular to a substrate surface. Notes:
1. See Fig. 20a.
2. When the alignment of the director in a homeotropic alignment deviates from the perpendicular, the alignment is said to be a pretilted homeotropic alignment; the pretilt angle is the deviation from 90°.
3. Surface pretilt is the deviation angle of the director away from the surface. It is used to control the threshold voltage and affects viewing angles.
4.4 planar alignment
homogeneous alignment
A molecular alignment in which directors lie parallel to a substrate surface. Note: See Fig. 20b.
4.5 uniform planar alignment
A molecular alignment in which the director is parallel to a substrate surface. Notes:
1. See Fig. 20c.
2. Sometimes a uniform planar alignment is called a “uniform homogeneous alignment”. The latter term is not recommended.
4.6 twist alignment
A molecular alignment for which the director rotates in a helical fashion when passing between two substrate surfaces having molecules in uniform planar alignments.
Notes:
1. See Fig. 21. The length of a line in Fig. 21 indicates the length of a director projected onto the plane of the page.
2. The orientation of the directors on the upper and lower substrate surfaces are usually mutually orthogonal, and hence the directors undergo a 90°twist over the thickness of the liquid-crystal layer.
4.7 defect
A nonuniform molecular alignment that cannot be transformed into a uniform alignment without creat-ing other defects.
Notes:
1. Dislocations and disclinations are major types of defects in liquid crystals.
2. Three elementary types of defects may be distinguished in liquid crystals. They are point, line, and wall defects.
3. A discontinuity in the structure (or in the mathematical function describing the structure) is con-sidered as a singularity; in many cases, a defect can be regarded as a singularity.
4.7.1 dislocation
A discontinuity in a regular molecular positional arrangement. Note: Dislocations are found mainly in solid crystals.
4.7.2 disclination
A defect along a line in the regular orientation of directors. Notes:
1. Disclinations are responsible for some optical textures seen with a polarizing microscope, such as the schlieren texture formed by disclination lines in nearly vertical orientations, whose projec-tions are seen as dark points with two or four emerging dark stripes or brushes (see Definition 4.9.2).
2. Disclinations are defects in molecular orientational order in contrast to dislocations that are defects in molecular positional order.
4.8 optical texture
An image of a liquid-crystal sample seen with a microscope, usually with crossed polarizers.
Note: An optical texture results from surface orientation of the directors at the boundaries of the sample and by defects formed in the sample.
4.9 nematic textures
4.9.1 nematic droplet
A spherical droplet that forms during a transition from an isotropic phase to a nematic mesophase. It has characteristic textures that depend on the droplet size and the director orientation at the nematic-isotropic interface.
Note: Nematic droplets display a texture characteristic of a nematic mesophase since they occur nowhere else.
4.9.1.1 bipolar droplet texture
A texture with two point defects at the poles of a nematic droplet. Notes:
1. A pole is the position of the extreme of a director in a droplet. 2. The point defects are called boojums.
3. A bipolar droplet texture occurs when the director lies in the plane of a nematic-isotropic inter-face.
4.9.1.2 radial droplet texture
A texture with one point defect at the center of a nematic droplet. Notes:
1. The point defect usually forms when the director is normal to the nematic-isotropic interface. 2. The radial droplet texture shows four dark brushes located in the regions where the director is in
the polarization plane of either the polarizer or the analyzer.
4.9.2 schlieren texture
A texture observed for a flat sample between crossed polarizers, showing a network of black brushes connecting centers of point and line defects.
Notes:
1. The black brushes are also called black stripes or schlieren brushes.
2. Black brushes are located in regions where the director lies in the plane of polarization of either the polarizer or the analyser.
3. Schlieren textures observed in nematic samples with planar alignment show defect centers with two or four emerging brushes. Schlieren textures in nematic samples with tilted alignments show centers with four brushes; centers with two brushes are caused by defect walls.
odd-mem-bered liquid-crystal dimers (see Definition 2.11.2.9, Note 5) has schlieren textures with two or four brushes.
4.9.2.1 nucleus
The center of a point or line defect from which black brushes originate when a liquid crystal is observed between crossed polarizers.
Note: A nucleus can indicate either the end of a disclination line terminating at the surface of a sample or an isolated defect.
4.9.2.2 disclination strength Recommended symbol: s
The number of rotations by 2πof the director around the center of the defect.
Notes:
1. s is positive when the brushes turn in the same direction as the polarizer and analyzer when they are rotated together, and negative when they turn in the opposite direction.
2. s can be an integer or half-integer since in nematics the directors +n and –n are not distinguish-able.
3. The angular distribution φof the director around a defect in a nematic planar texture, in the X-Y projection, can be expressed in terms of the polar angle θr
φ= sθr + φo
where θrrepresents the angular polar coordinate of a given point with respect to the disclination center, φis the angle that the local director axis at that point makes with the θr = 0 axis, and φois a constant (0< φo<2π) (See Fig. 22).
The product sθryields the angle by which the director turns on a closed curve around the disclination center. If a complete circuit is made around the center of an s = ±1/2 disclination, the director rotates by π. For s = ±1 a similar circuit yields a total director rotation of 2π. So, s = ±1/2 defines a π-line disclination and s = ±1 defines a 2π-line disclination.
4. Director alignments for point defects with different values of s are illustrated in Fig. 23.
Fig. 22 (a) Identification of the angles φandθrused to describe a disclination. (b) Director arrangement of an
s = +1/2 singularity line. The end of the line attached to the sample surface appears as the point s = +1/2
4.9.3 threaded texture
A texture with π-line disclinations which lie essentially parallel to the surfaces between which a sam-ple is placed, with the ends of the lines attached to the surfaces and the other parts of the lines moving freely in the liquid crystal, appearing as thin thread-like lines.
4.9.3.1 surface disclination line adhering thread
A thick, thread-like disclination line anchored along its length to the upper or the lower of the surfaces between which a sample is placed.
4.9.4 marbled texture
A texture consisting of several areas with different director orientations.
Note: On observing a sample with a marbled texture between crossed polarizers, the interference color is essentially uniform within each individual area, indicating an essentially homogeneous region.
4.10 smectic textures
4.10.1 bâtonnet
A droplet, usually nonspherical, of a smectic phase nucleating from an isotropic phase.
4.10.2 focal-conic domain
A domain formed by deformed smectic layers that fold around two confocal line defects preserving equidistance of structural layers everywhere except in the vicinity of the defect lines.
Notes:
1. See Fig. 24. The confocal line defects may be an ellipse and a hyperbola or two parabolae. 2. The smectic layers within a focal-conic domain adopt the arrangement of Dupin cyclides, since
as in these figures there appear concentric circles resulting from the intersection of ellipses and hyperbolae. They also have the distinctive property of preserving an equal distance between them. 3. A focal-conic domain built around an ellipse and an hyperbola is the most common type of defect in thermotropic smectic A phases. The hyperbola passes through a focus of the ellipse and the ellipse passes through the focus of the hyperbola (see Fig. 24).
4. In a particular limiting case of an ellipse-hyperbola focal-conic domain, the ellipse becomes a straight line passing through the center of a circle.
Fig. 23 Schematic representation of the director alignments at disclinations with different values of s and
φo; s = ±1⁄
5. A focal-conic domain built around two confocal parabolae is called a parabolic focal-conic domain.
6. At any point inside a focal-conic domain, the director is oriented along the straight line drawn through the point and the two defect lines (ellipse and hyperbola or two parabolae or circle and straight line). See for examples BD, BC, and BO in Fig. 24.
4.10.3 polygonal texture
A texture composed of focal-conic domains of the ellipse-hyperbola type with visible ellipses, or parts of ellipses, located at the boundary surfaces.
Notes:
1. See Figs. 25a and 25b.
2. One branch of the hyperbola (either above or below the plane of the ellipse) is usually missing in the polygonal texture.
3. Neighboring domains form a family with a common apex where the hyperbolae of these domains join each other. This common point is located at the surface that is opposite to the surface con-taining the ellipses (see Fig. 26). Each family is bounded by a polygon formed by hyperbolic and elliptical axes; these are parts of focal-conic domains that provide a smooth variation of smectic layers between the domains of different families. These domains are the tetrahedra in Fig. 26. 4. The smectic layers pass continuously from one focal-conic to the next.
Fig. 24 Dupin cyclide and perfect focal-conic domain construction: (a) vertical section showing layers of the
structure; thick lines indicate the ellipse, hyperbola, Dupin cyclide, and central domain; (b) focal-conic domain showing structural layers with a representation of the arrangement of the molecules within one of them.
Fig. 25 Arrangement of a smectic A polygonal texture: (a) general view of the focal-conic domains filling space
4.10.4 focal-conic, fan-shaped texture
A texture formed partly by focal-conic domains with their hyperbolae lying in the plane of observation. Notes:
1. See Fig. 27.
2. The layers are aligned almost normal to the sample surfaces. The regular sets of hyperbolae are called “boundaries of Grandjean”; they serve as limiting surfaces between domains with dif-ferent director orientations.
5. PHYSICAL CHARACTERISTICS OF LIQUID CRYSTALS
General Note: In this section the director n is treated mathematically as a unit vector, with components n1, n2, n3along space-fixed axes X1, X2, X3.
5.1 order parameter
Recommended symbol: <P2>
A parameter characterizing the long-range orientational order with reference to the director, with
<P2> = (3 < cos2β> –1) / 2
where βis the angle between the molecular symmetry axis and the director and < > denotes an ensem-ble average.
Fig. 26 Elements of a smectic A complex polygonal texture. Upper surface: one polygon with four ellipses.
Lower surface: two polygons. The whole space may be divided into three pyramids (ABCDK, AEHKJ, BFGKJ) and three tetrahedra (ABJK, ADHK, BCGK).
Fig. 27 (a) Illustrating an arrangement of confocal ellipses and hyperbolae. The directors become parallel near
Notes:
1. <P2> characterizes long-range molecular order.
2. For rod-like molecules, the order parameter of the effective molecular symmetry axis at the nematic-isotropic transition is about 0.3 and can increase to about 0.7 in the nematic mesophase. 3. Molecules which constitute nematogens are not strictly cylindrically symmetric and have their ori-entational order given by the Saupe ordering matrix which has elements Sαβ= (3<|α|β>– δαβ)/2, where lα and lβare the direction cosines between the director and the molecular axes αand β, δαβis the Kroenecker delta, and α,βdenote the molecular axes X, Y, Z.
4. The constituent molecules of a nematogen are rarely rigid, and their orientational order is strict-ly defined, at the second-rank level, by a Saupe ordering matrix for each rigid subunit.
5. Even for molecules with cylindrical symmetry, <P2> does not provide a complete description of the orientational order. Such a description requires the singlet orientational distribution function, which can be represented as an expansion in a basis of Legendre polynomials PLcosβ, with L an even integer. The expansion coefficients are proportional to the order parameters <PL> of the same rank. It is these order parameters that provide a complete description of the long-range ori-entational order.
5.2 distortion in liquid crystals
Recommended symbol: a
A deformation leading to a change in the director, where the distortion is described by a tensor of third rank
in which the initial orientation of the director n is chosen as the 3-axis; i = 1,2; j = 1,2,3; n3= 1, niis the ith component of the director n, and xjis a coordinate on axis Xj.
5.2.1 splay deformation
Recommended abbreviation: S-deformation
Deformation in a direction normal to the initial director, n, characterized by div n≠0. Notes:
1. See Fig. 28 and Definition 5.3.
2. A splay deformation is described by the nonzero derivatives n3(∂n1/∂x1) and n3(∂n2/∂x2), where the symbols are defined in Definition 5.2.
a3ij=n3
(
∂ ∂ni xj)
Fig. 28 Schematic representation of a splay deformation: (a) changes in the components of the director n,
5.2.2 bend deformation
Recommended abbreviation: B-deformation
Deformation in the direction of the initial director, n, characterized by n ×rot n≠0. Notes:
1. See Fig. 29 and Definition 5.3.
2. The degree of bending is given by the component of rot n perpendicular to n.
3. A bend deformation is described by the nonzero derivatives n3(∂n1/∂x3) and n3(∂n2/∂x3), where the symbols are defined in Definition 5.2.
5.2.3 twist deformation
Recommended abbreviation: T-deformation
A torsional deformation of a planar-oriented layer in the direction of the initial director, n, character-ized by n⋅rot n≠0.
Notes:
1. See Fig. 30 and Definition 5.3.
2. The degree of twisting is given by the component of rot n parallel to n.
3. A twist deformation is defined by the nonzero derivatives n3(∂n1/∂x2) and –n3(∂n2/∂x1), where the symbols are defined in Definition 5.2.
Fig. 29 Schematic representation of a bend deformation: (a) changes in the components of the director,
n defining the orientation change; (b) bend deformation of an oriented layer of a nematic liquid crystal.
Fig. 30 Schematic representation of the twist deformation: (a) changes in the components of the director n,
